When Sunshine and Milk Aren’t Enough

Many kids don’t get enough vitamin D, and that is not great for their little, growing bodies.

Sunscreen, so valuable in blocking dangerous UV rays, reduces the skin’s ability to make vitamin D

Photo illustration by Slate. Photo by ampak/Thinkstock.

The summer sun is almost upon us, which means my kids will be getting no vitamin D whatsoever, because I’m going to slather sunscreen on them like cream cheese on bagels. Considering that my son doesn’t like milk and my daughter spills more than she drinks, I’ve wondered just how much vitamin D they get and whether it’s enough.

Turns out a lot of parents should be concerned about this. Many American kids are not getting enough vitamin D, which could be damaging their bones or worse. A 2009 study published in the journal Pediatrics analyzed data from the U.S. National Health and Nutrition Examination Survey and found that nearly 1 out of 5 American kids aged 1 to 11 have blood levels of vitamin D below the 50 nanomols per liter (nmol/L) recommended by the American Academy of Pediatrics. That is a pretty conservative benchmark: The Canadian Paediatric Society recommends that kids maintain a higher vitamin D blood level of 75 nmol/L—yet a whopping 69 percent of U.S. kids do not meet this guideline. A 2012 study reported similar numbers among healthy-weight kids and found that obese kids were even more likely to be vitamin D deficient. (Darker skinned kids, whose bodies don’t make vitamin D as efficiently from sunlight, as well as obese kids tend to be most at risk.)

So is the solution to give kids vitamin D supplements? How do you know if your wee ones are getting enough or too much—and why does vitamin D matter, anyway?

Let’s start with some science, because vitamin D is controversial. It’s not that doctors disagree over whether it’s important—it’s that they disagree over how many things it’s important for and how much people need. Everyone acknowledges that vitamin D boosts calcium absorption in the gut and builds strong bones; without it, bones get brittle and misshapen. Rickets, a disease of vitamin D deficiency, can lead to chronic pain and short stature.

What’s contentious is the research that links low vitamin D to a cadre of other conditions: cancer, heart disease, diabetes, asthma, respiratory tract infections, psoriasis, multiple sclerosis, and rheumatoid arthritis, among others. Studies typically find that people with high vitamin D blood levels are less likely than people with low vitamin D levels to develop these conditions; the problem is that these types of observational studies cannot tease out cause and effect. Perhaps vitamin D doesn’t reduce the risk of these diseases directly—maybe instead, higher vitamin D levels and lower disease risks are both caused of some third, as yet unknown, factor. Or maybe blood vitamin D is an indicator of good health rather than a cause of it.

The best way to figure out whether something (like vitamin D) causes or prevents another thing (like cancer) is through randomized controlled clinical trials, which would involve giving vitamin D to some randomly chosen people but not to others and then seeing whether the high-vitamin group is less likely to get sick than everyone else. But these trials are exorbitantly expensive and can take decades—and wealthy pharmaceutical companies, which typically underwrite such trials, aren’t interested in doing them because they can’t patent vitamins and sell them exclusively. “There’s no profit potential in this, so it’s ignored,” explains Bruce Hollis, the director of pediatric nutritional sciences at the Medical University of South Carolina, who studies vitamin D. This is too bad, because it would be valuable to know whether vitamin D really can reduce the risk of these life-or-death conditions.

That said, some small trials do suggest that vitamin D, in addition to building strong bones, can help certain health issues in kids. A 2015 systematic review and meta-analysis of five clinical trials found that vitamin D supplements (between 500 and 2,000 international units, or IU, daily) reduce the frequency of asthma attacks in asthmatic kids by 59 percent. And a 2012 systematic review and meta-analysis of five trials reported that vitamin D lowers children’s risk for respiratory tract infections such as colds, flu, and pneumonia. But again, most of the trials included in these reviews were small, so more research is crucial.

Still, given that, at the very least, vitamin D is important for bone health, it’s essential for kids to get enough. And there are two ways for a body to get it: sunshine and diet. The problem with the first is that most kids aren’t getting unfettered sun exposure these days—they’re not outside as much as they used to be, they’re not running around naked very frequently, and they aren’t frolicking without sunscreen. (Sunscreen reduces the skin’s ability to make vitamin D.) The decision to use sunscreen is undoubtedly a smart one, as rates of melanoma have been increasing in recent decades, particularly among whites. But the downside is that sun-induced vitamin D is on the decline. A 2013 study published in JAMA Pediatrics analyzed how various factors contributed to children’s vitamin D levels and concluded that while outdoor time does provide some vitamin D, its contribution is small: Every hour of outdoor play increased blood vitamin D levels in kids by 0.75 nanomols per liter, whereas every cup of cow’s milk they drank contributed 4 nanomols per liter—more than five times as much.

Milk is, indeed, the main way kids get vitamin D today. Fatty fish and fish oils are also high in the vitamin, but kids don’t usually eat enough to get what they need. And nut- and plant-based milks won’t cut it, either. In a 2016 study, researchers found that kids who drank these alternative milks had much lower vitamin D levels than kids who drank cow’s milk. Though many brands claim they are fortified with Vitamin D, they may not actually be adding as much as they say, because the claims aren’t tightly regulated. (Goat’s milk, however, does seem to provide about the same amount of vitamin D as cow’s milk, so that’s an OK alternative.)

In light of how few foods reliably provide vitamin D and how little kids get from the sun, the AAP released guidelines in 2008 recommending that kids who drink less than one liter of milk a day take 400 IU of vitamin D supplements daily. But here’s where things get tricky. Clearly, the AAP’s implication is that if you want little Annabelle to avoid taking supplements, she should drink a whopping liter of cow’s milk a day. Except, well, she shouldn’t. “Young children shouldn’t be drinking that much cow’s milk,” says Jonathon Maguire, a pediatrician and researcher at St. Michael’s Hospital in Toronto. That’s because cow’s milk interferes with iron absorption. In a 2013 study published in the journal Pediatrics, Maguire and his colleagues reported that kids who drank more than two cups of cow’s milk a day suffered dramatic drops in blood iron levels. So if your kid starts drinking a liter of milk a day, yes, she might get enough vitamin D, but she won’t get enough iron.

Given this conundrum, I asked Maguire and Hollis for their suggestions. Maguire says that based on his research, two cups of cow’s milk a day, which provides a total of 200 IU of vitamin D, is enough to stave off rickets in most light-skinned kids. (Darker-skinned kids should also take a supplement.) But Hollis is less conservative and recommends supplements for everyone. “My recommendation, and what I practice within my own family, is that children need, from the age of 1 or 2, at least 1,000 IU of vitamin D a day—and progress up to maybe 4,000 a day as they get in their teens,” he says. (His higher recommendations are largely based on the body of research linking high vitamin D levels to a lower risk of chronic diseases as well as other studies linking vitamin D to better immune function.) In the middle of these two poles sits the AAP’s recommendation, which is for kids to take 400 IU of vitamin D supplements if they aren’t drinking boatloads of milk. To make things even more confusing, the Endocrine Society, an organization comprised of U.S. endocrinologists, recommends that kids older than 1 get at least 600 IU of vitamin D a day, and the Institute of Medicine has also set 600 IU as the recommended daily allowance for vitamin D in kids older than 1. (Vitamin D is important throughout adolescence, too—when your son shoots up a foot, his bones have to grow fast to keep up, so these guidelines don’t just apply to young kids.)

In light of all this conflicting advice, what’s a parent to do? If your kid is light-skinned, drinks two cups of cow’s milk a day, and also gets some sun exposure and occasionally eats fatty fish like salmon, he’ll probably be fine. But if you want to be absolutely certain he’s meeting his body’s vitamin D needs, or you have a child with dark skin or who doesn’t drink milk or play much outside, supplementing is a good idea. In fact, gummies and chewable vitamins that provide vitamin D3 are a good bet—ConsumerLab.com, a company that independently tests health and nutrition products, just released a report highlighting recommended brands—and it’s ideal for kids to take them with their largest meal, because more of the vitamin will be absorbed. Highly concentrated vitamin D drops are also an option, but the AAP cautions that they must be used carefully because of the risk of overdose, which can cause calcium to build up in the blood and lead to high blood pressure, nausea, and constipation. (As for what’s too much: The Institute of Medicine says that 1-to-3-year-olds should not consume more than 2,500 IU of vitamin D daily; 4-to-8-year-olds shouldn’t exceed 3,000 IU; and kids 9 and older should not get more than 4,000 IU a day.) I have decided to give both my kids 800 IU in vitamin D gummies each day, and it feels like a win-win. To them, it’s dessert. To me, it’s supplemental health insurance, and I can still coat them in SPF 50 with joyful abandon.